feat: enforce boundaries by simulating pure eval #7
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With this, we have essentially achieved NixOS/nix#6530 in pure nix. The only remaining "impurity" in this sense would be the path passed to the composer function, but this is actually a good thing, since it doesn't eagerly copy everything to the nix store like flakes, but instead only makes pure copies of files when they are actually accessed. If the path doesn't exist on the file-system we will immediately fail anyway, so it's "impurity" is irrelevant. There will be no way for the user to escape the module system with these new enforcements, so that means that we basically have a totally pure nix evaluation without the high cost of having to copy the entire repo into the /nix/store eagerly.
While we don't want to encourage it, it would be helpful to ease adoption if users could call this module function from inside their flake.nix. The only thing preventing that was the path referencing becoming a pure /nix/store path reference implicitly (via the flake) and so failing because of the string context. Fix this by simply removing the string context. This isn't actually unsafe since if the absolute path literal refers to a store path and is ever coerced back to a string, nix will readd the context automatically.
blaggacao
suggested changes
Aug 5, 2024
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With this, we have essentially achieved NixOS/nix#6530 in pure nix. In fact, we are a bit "more pure" since we don't allow arbitrary imports (instead all code must be a module member somewhere) and we don't allow referencing arbitrary paths (modules can only access the (non-nix) files in their own directory through "${mod}"`. We also disable the builtin fetchers to encourage users to declare their dependencies statically in the manifest, and also because those fetchers block evaluation (and therefore builds).
The only remaining "impurity" in this sense would be the path passed to the composer function, but this impurity is actually a good thing, since it doesn't eagerly copy everything to the nix store like flakes, but instead only makes pure (/nix/store) copies of files when they are actually accessed.
If the path doesn't exist on the file-system we will immediately fail anyway, so it's "impurity" is irrelevant. With these added restrictions, there will be no way for the user to escape the module system, so that means that we basically have a totally pure nix evaluation without the high cost of having to copy the entire repo into the /nix/store eagerly.